1. Field of the Invention
The present invention relates generally to the formation of composite structures and, more particularly, to the formation of the stiffeners or other reinforcing members associated with such composite structures.
2. State of the Art
In the fabrication of composite structures, structural members are often attached to a skin to provide reinforcement of the skin. Such structural members may include, for example, ribs, spars or frames configured to be attached to the skin of the composite structures. Such structural members may also include substantially elongated stiffening members often referred to as stringers or stiffeners. The stringers or stiffeners may be formed to exhibit various cross-sectional geometries including configurations such as I-beams, C-shapes or channels, J-shapes, Z-shapes, L-shapes or angles, omega shapes or what is often referred to as a hat shape or a hat channel. A stiffener or stringer exhibiting a cross-sectional geometry or profile of a hat essentially includes a cap member having a pair of web members, one web member extending from each end of the cap member at a defined angle relative thereto, and a pair of flange members with one flange member extending from each web member at a defined angle relative to the associated web member. In the cross-sectional geometry of some hat stiffeners, the flange members may be configured to be substantially parallel with the cap member.
A current method of forming composite hat stiffeners, as well as stiffeners exhibiting other cross-sectional geometries, includes laying up composite plies by hand, one at a time, over a mold, mandrel or other similar tool to form a laminate structure. Upon laying up every two to three plies, the laminate structure needs to be compacted or debulked. This is conventionally accomplished by vacuum debulking wherein a vacuum bag is placed over the laminate structure and a vacuum applied to the structure by way of the bag. Often, heat may be applied to assist in the debulking process and in an attempt to further compact the laminate structure. Each vacuum debulk performed on the laminate structure represents a time-consuming process. In forming the laminate structure, multiple vacuum compactions may need to take place upon the building up of layers to form the laminate structure. However, even with multiple vacuum debulks being performed on a given laminate structure, the laminate structure may still undesirably exhibit a significant amount of bulk.
Once all of the plies have been positioned and the laminate structure has been initially formed (including the process of subjecting the laminate structure to vacuum debulking processes), the laminate structure may be cured and subsequently attached to a skin structure, such as with adhesive, or may be cocured (cured concurrently) with the skin structure, thereby bonding the two components together. Curing of the laminate structure is conventionally accomplished by placing the laminate structure in a cure mold and subjecting it to a high pressure and high temperature such as in an autoclave or similar environments
When the laminate structures are placed in a cure mold, because they still exhibit a substantial amount of bulk, they may not fit properly within the mold. Furthermore, while any remaining bulk exhibited by the laminate structure tends to be driven out during the curing process, such as in an autoclave, there is little, if any, slip allowed between the plies of the laminate structure and, as a result, ply bridging and ply wrinkling will often occur within the cured or partially cured laminate structure.
While it is possible to obtain structure with low bulk characteristics by subjecting the structures to multiple hot debulks under autoclave pressure, such is a very time-consuming and expensive process. Additionally, such a process may shorten the working life of the laminate structure due to the repeated subjection thereof to high temperatures. Furthermore, such an aging process can hinder the ability of the laminate structure to be cocured with a mating skin or other structure.
In addition to the issues of obtaining a low bulk structure, the conventional process of forming composite reinforcing structures by hand has other limitations. For example, the method of forming elongated reinforcing structures by hand poses difficulties in obtaining shapes which, besides exhibiting a desired cross-sectional geometry, also exhibit bends along a longitudinal axis or twist about the longitudinal axis of the structure. Such features are difficult to accomplish, in part, because it is difficult to manipulate the plies by hand to conform to such bends and/or twists without introducing additional wrinkles or waves into the laminate structure being formed. Furthermore, the manipulation of plies by hand is an extremely time-consuming and labor-intensive process, thereby increasing the cost of manufacturing such parts.
Various attempts have been made to provide a process which provides elongated reinforcing structures without the various limitations which are presented by the conventional process of laying up individual composite plies by hand. For example, pultrusion is a process which has been used to form plastic materials, including fiber-reinforced plastic composite materials, into structures exhibiting a desired cross-sectional geometry or profile. An example of such a pultrusion process is set forth in U.S. Pat. No. 5,026,447 issued to O'Connor. O'Connor teaches the pulling of an elongated body of reinforced thermoplastic material through a plurality of dies. The dies are operated independently of each other such that any combination of the dies may be selected to impart a cross-sectional geometry to a portion of the elongated body. The process of O'Connor purportedly allows the manufacture of an elongated thermoplastic member which may exhibit varied cross-sectional geometries along the length thereof However, as will be recognized by those of ordinary skill in the art, there are various limitations associated with the process of pultrusion.
For example, pultrusion is conventionally associated with materials utilizing a thermoplastic resin. The use of thermosetting resins may cause a buildup of material on the dies and considerable inefficiencies in forming the desired cross-sectional shape of the pultruded member. Additionally, it is often difficult to obtain a fiber orientation in the resultant member which varies significantly from the longitudinal axis of the formed member (i.e., along the direction which the member is pulled through the die or dies). Furthermore, because the process involves forming the member by pulling a plurality of fibers through a die and then cooling the member until the resin substantially resolidifies, such a process is generally only effective for forming straight or linear members and may not be effective in forming members exhibiting a cross-section which does not vary substantially in terms of overall cross-sectional area. It is also noted that the dies used in pultrusion are generally expensive to manufacture and numerous dies are required if it is desired to produce elongated members of more than one cross-sectional geometry.
Other processes for forming elongated thermoplastic members include, for example, U.S. Pat. No. 5,891,379, issued to Bhattacharyya et al., and U.S. Pat. No. 5,182,060, issued to Berecz. Bhattacharyya discloses a process of forming fiber-reinforced plastic material into a desired shape which includes heating the material to a temperature above the melting temperature of the thermoplastic resin or matrix material. The heated material is cooled below the melting temperature but still maintained at a temperature which is above the recrystallization temperature of the thermoplastic material, and then passed through a plurality of roll-forming dies in order to produce a desired shape. The shaped material is then further cooled so that the fiber-reinforced plastic material will retain the shape imposed thereto by roll-forming dies. Berecz discloses a process of continuously forming a thermoplastic composite shape including heating the unidirectional tape or woven cloth, passing the heated material through a set of rollers, and then passing the heated material through a matched metal die which acts as a rapidly reciprocating punch to form the final shape.
While the processes taught by Bhattacharyya and Berecz appear to allow improved control of the fiber orientation in the resultant part over a conventional pultrusion process, the disclosed processes appear to be limited to the use of materials comprising thermoplastic resins including subjecting the materials to temperatures at or above melting temperatures of the resin prior to forming the desired cross-sectional geometries. As will be appreciated by those of ordinary skill in the art, the use of thermoplastic resins provides considerable flexibility in being able to melt, or substantially melt, the resin and subsequently reheat the resin in order to reshape/rework the member and/or to adhere the member to another structure by means of contacting the other structure with the melted or substantially melted resin material.
However, such a process is not amenable to the formation of reinforcing or structural members comprising thermosetting materials since, if the thermosetting resin is heated above a specified temperature to allow the resin to readily flow and thereby assist in forming the composite material into a specified cross-sectional geometry, the thermosetting resin will cross-link and cure. Once the reinforcing member is cured, it will not be possible to perform any subsequent rework of the member, nor will the member be able to be bonded to another structure through cocuring.
For example, U.S. Pat. No. 5,043,128 to Umeda discloses a process of forming an elongated composite member utilizing a thermosetting resin which includes feeding a plurality of preimpregnated carbon fiber sheets of material through a pair of shaping rollers and into a heating and press forming device. The heating and press forming device includes a heating die and a press punch configured to engage the heating die. The sheets of material are temporarily stopped within the heating and press forming device and pressed by the punch against the heating die. The sheets of material are, thus, simultaneously pressed and heated, resulting in the thermosetting, or curing, of the sheets of material into the desired shape. As noted above, a process of forming a structural member which includes the curing of a thermosetting resin prevents any subsequent reworking of the member and/or any cocuring of the structural member with, for example, a composite skin or other structural member. Thus, in order to form a structural member exhibiting a desired cross-sectional geometry from a composite material comprising a thermosetting resin which is not fully cured, methods such as that described above wherein multiple plies are laid up by hand over a mandrel or mold are still utilized.
In view of the shortcomings in the art, it would be advantageous to provide an apparatus and a method for forming elongated reinforcing or structural members of a material comprising a thermosetting resin which enables the member to exhibit a desired cross-sectional geometry without fully curing the member.